The present invention relates to a LADAR transmitting and receiving apparatus, and, more particularly, to a compact LADAR receiving apparatus with enhanced signal-to-noise performance and bandwidth.
According to Wikipedia, Lidar (Light Detection And Ranging or Light Imaging, Detection, And Ranging) measures distance by illuminating a target with a laser light. Lidar may be used for a variety of purposes including high-resolution maps (including airborne laser swath mapping (ALSM)) and laser altimetry. Lidar is alternately referred to as laser scanning or 3D scanning; with terrestrial, airborne and mobile applications. Laser Detection And Ranging (LADAR) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. LADAR may be used in a variety of contexts for elastic backscatter light detection and ranging (LIDAR) systems. Although the acronym LADAR is usually associated with the detection of hard targets and the acronym LIDAR is usually associated with the detection of aerosol targets, there has been no real standard on their use and both acronyms may be used interchangeably to describe the same laser ranging system. Accordingly, as used herein, the terminology LIDAR means LADAR and vice versa.
LADAR systems typically operate in the ultraviolet, visible, or near infrared spectrums, which gives a compact LADAR the ability to image a target at a high spatial resolution and allows LADAR systems to be made more physically compact.
As reported in U.S. Pat. No. 8,081,301, in order for a LADAR system target to reflect a transmitted electromagnetic wave, an object needs to produce a conductive or dielectric discontinuity from its surroundings. At radar frequencies, a metallic object produces a conductive discontinuity and a significant specular reflection. However, non-metallic objects, such as rain and rocks produce weaker dielectric reflections, and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies. Lasers provide one solution to this problem regarding non-metallic detection. The beam power densities and coherency of lasers are excellent. Moreover, the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 μm to around 250 nm. At such wavelengths, the waves are reflected very well from small objects such as molecules and atoms. This type of reflection is called diffuse “backscattering.” Both diffuse and specular reflection may be used for different LADAR applications.
Some prior art LADAR systems have transmitter and receiver functions that rely on a co-axial or mono-static optical system that comprises a complex assembly of beam splitters, polarizers, and steering mirrors that is very difficult to align, prone to losing alignment, subject to narcissus, and requiring excessive space for a compact LADAR system. As reported in U.S. Pat. No. 8,081,301, compact LADAR systems have generally been flawed by one or more factors including, low pixelization, insufficient range or range resolution, image artifacts, no daylight operation, large size, high power consumption, and high cost. Prior art systems may use a wide bandwidth photo detector/amplifier system with a small detector, and a low shunt capacitance, leading to a low signal-to-noise ratio or small field-of-view.